1. Field of the Invention
The present invention relates to metal-carbon composite powders and to methods for producing such powders, as well as products and devices incorporating the composite powders. The powders are preferably produced by a spray conversion process.
2. Description of Related Art
Many product applications require metal-carbon composite powders. Such composite powders should have one or more of the following properties: high purity; controlled crystallinity; small average particle size; narrow particle size distribution; spherical particle morphology; controlled surface chemistry; controlled surface area; and little or no agglomeration of particles. Examples of metal-carbon composite powders requiring such characteristics include, but are not limited to, those useful in electrocatalyst applications such as fuel cells and batteries, as well as in conductive pastes and inks.
With the advent of portable and hand-held electronic devices and an increasing demand for electric automobiles due to the increased strain on natural resources there is a need for rapid development of high performance, economical power systems. Such power systems require improved means for both energy storage, achieved by use of batteries, and energy generation, achieved by use of fuel cells. Batteries can be subdivided into primary (non-rechargeable) and secondary (rechargeable) batteries.
Fuel cells are electrochemical devices which are capable of converting the energy of a chemical reaction into electrical energy. The electrical energy is produced without combustion and creates virtually no pollution. Fuel cells are unlike batteries because fuel cells convert chemical energy to electrical energy as the chemical reactants are continuously delivered to the fuel cell. When the fuel cell is off, it has zero electrical potential. As a result, fuel cells are typically used to produce a continuous source of electrical energy and compete with other forms of continuous electrical energy production such as the combustion engine, nuclear power and coal-fired power stations. Different types of fuel cells are categorized by the electrolyte used in the fuel cell. The five main types of fuel cells are alkaline, molten carbonate, phosphoric acid, solid oxide and proton exchange membrane (PEM) or solid polymer fuel cells.
In fuel cells, gases are often used as a source of chemical energy which is converted to electrical energy. One of the critical requirements for these energy devices is the efficient catalytic conversion of the reactants to electrical energy. A significant obstacle to the wide-scale commercialization of such devices is the need for superior electrocatalyst materials for this conversion process.
A PEM fuel cell stack is comprised of hundreds of membrane electrode assemblies (MEA's). An MEA includes a cathode and anode, each constructed from, for example, carbon cloth. The anode and cathode sandwich a proton exchange membrane which has a catalyst layer on each side of the membrane. Power is generated when hydrogen is fed into the anode and oxygen (air) is fed into the cathode. In a reaction catalyzed by a platinum-based catalyst in the catalyst layer, the hydrogen ionizes to form protons and electrons. The protons are transported through the proton exchange membrane to a catalyst layer on the opposite side of the membrane where another catalyst, typically platinum or a platinum alloy, catalyzes the reaction of the protons with oxygen to form water. EQU Anode: 2H.sub.2 .fwdarw.4H.sup.+ +4e.sup.- EQU Cathode: 4H.sup.+ +4e.sup.- +O.sub.2 .fwdarw.2H.sub.2 O EQU Overall: 2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O
The electrons formed at the anode are routed to the cathode through an electrical circuit which provides the electrical power.
The critical issues that must be addressed for the successful commercialization of fuel cells are cell cost, cell performance and operating lifetime. In terms of fuel cell costs, current fuel cell stacks employ MEA's containing unsupported platinum black electrocatalysts with a loading of about 4 milligrams of platinum per square centimeter on each of the anode and cathode. When this loading is compared to a typical cell performance of 0.42 watts per square centimeter, then 19 grams of platinum per kilowatt is required. It is clear that a significant cost reduction in the electrocatalyst is necessary for these cells to become economically viable. However, reducing the amount of precious metal is not a suitable solution because there is also a strong demand for improved cell performance. For automotive applications, improved power density is critical whereas for stationary applications, higher voltage efficiencies are necessary. The major technical challenge continues to be improved cathode electrocatalyst performance with air as the oxidant.
A type of battery which utilizes a similar principle is the zinc-air battery, which relies upon the redox couples of oxygen and zinc. Zinc-air batteries are advantageous since they consume oxygen from the air as a fuel, contain no toxic or explosive constituents and operate at one atmosphere of pressure. Zinc-air batteries typically operate by adsorbing oxygen from the air where it is reduced using an oxygen reduction catalyst. As the oxygen is reduced, zinc metal is oxidized. The two half-reactions of a zinc-air battery during discharge are: EQU Cathode: O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.- EQU Anode: 2Zn.fwdarw.2Zn.sup.2+ +4e.sup.- EQU Overall: 2Zn+O.sub.2 +2H.sub.2 O.fwdarw.2Zn(OH).sub.2
Zinc-air batteries can be primary batteries or secondary batteries. Although zinc-air batteries consume oxygen as a fuel, they are typically not considered fuel cells because they have a standing potential without a fuel source. Zinc-air cells absorb oxygen from the air on the air electrode during discharge and release air out of the cell during recharge.
Typically, air electrodes (cathodes) are alternatively stacked with zinc electrodes (anodes) which are packaged in a container that is open to the air using small holes or ports. When the battery cell discharges, oxygen is reduced to O.sup.2- while zinc metal is oxidized to Zn.sup.2+. When all of the zinc has been oxidized, the secondary battery can be recharged where Zn.sup.2+ is reduced back to zinc metal.
The advantages of zinc air batteries over other rechargeable battery systems are safety, long run time and light weight. The batteries contain no toxic materials and can run as long as 10 to 14 hours, compared to 2 to 4 hours for most lithium-ion batteries. Zinc-air batteries are also very light weight, leading to good power density (power per unit of weight or volume), which is ideal for portable applications. The two major problems associated with zinc-air batteries, however, are limited total power and poor rechargeability/cycle lifetime.
In particular, power is becoming a major area of attention for battery manufacturers trying to meet the increased demands of modern electronics. Current zinc-air batteries can deliver sufficient power to permit the batteries to be used in specific low-power laptops and other portable devices that have relatively low power requirements. Most laptops and other portable electronic devices, however, require batteries that are able to provide a level of power that is higher than the capabilities of current zinc-air batteries.
The main reason for the low power of zinc-air batteries is believed to be related to the inefficiency of the catalytic reactions in the air electrodes. In zinc-air batteries, metal-carbon composite powders are used at the cathode to reduce the oxygen from the air to O.sup.2-. It is believed that poor accessibility of the catalyst and the local microstructural environment around the catalyst and adjoining carbon is important in the efficiency of oxygen reduction. See, for example, P. N. Ross et al., Journal of the Electrochemical Society, Vol. 131, pg. 1742 (1984).
Rechargeability is also a problem with zinc-air batteries. Current zinc-air technology can deliver safe, non-toxic and light weight batteries with very long run times. However, the batteries degrade in performance after a number of recharging cycles and therefore have a short cycle life. The short cycle life of zinc-air batteries is believed to be related to the catalyst used in the air electrodes. Specifically, it is believed that corrosion of the carbon used in these systems leads to a loss in capacity and hence, a decreasing discharge time. Control over the powder properties such as crystallinity, surface area and metal dispersion can enhance the performance of these batteries.
Methods for preparing noble metal electrocatalyst materials are known in the art. U.S. Pat. No. 4,052,336 by VanMonffoort et al. discloses a process for preparing an active noble metal catalyst on a carbon carrier, such as palladium on carbon, by adsorbing a salt of the metal onto the carbon, forming an oxide or hydroxide from the metal salt and reducing the oxide or hydroxide to a metal. The carbon support comprises porous active carbon particles having a widely varying particle size of less than 1 .mu.m up to 60 .mu.m. The catalyst comprises from about 0.1 to about 15 percent by weight of the noble metal. It is disclosed that the noble metal is deposited on the carbon carrier in the form of very small crystallites which have a high degree of catalytic activity per gram of noble metal.
U.S. Pat. No. 4,136,059 by Jalan et al. discloses a method for the production of electrochemically active platinum particles for use in fuel cell electrodes. The particles are formed by mixing chloroplatinic acid and sodium dithionite in water to provide a colloidal dispersion which is absorbed onto a support material (e.g. carbon black).
U.S. Pat. No. 4,482,641 by Wennerberg discloses a high surface area porous active carbon matrix containing a uniform dispersion of a metal. The material is formed by spray drying a carbon precursor and a metal precursor to form particles and then pyrolyzing the spray dried particles under an inert gas and in the presence of an alkali metal hydroxide. A preferred heating method for the pyrolyzation step is to heat using microwave heating. It is disclosed that the metal crystals have a size from about 5 to 30 angstroms and are disposed on active carbon having a cage-like structure.
U.S. Pat. No. 4,569,924 by Ozin et al. discloses a carbon-metal catalyst having an active metal such as silver deposited on the carbon substrate in a zero-valent, small cluster form. The catalyst is produced by vaporizing the metal under low vapor pressure conditions in an organic liquid solvent such that the metal dissolves in the solvent. The solvent is then contacted with carbon so that the complex diffuses onto the surface of the carbon and into the pores of the carbon. The carbon particles have a metal loading of 0.1 to 15 weight percent.
U.S. Pat. No. 4,652,537 by Tamura et al. discloses a process for producing a catalyst useful for converting carbon monoxide into carbon dioxide. The process includes contacting activated carbon with an aqueous solution of chloroplatinic acid, reducing the absorbed chloroplatinic acid to platinum with a reducing agent and decomposing the excess reducing agent. The catalyst preferably contains at least about 6 milligrams of platinum per gram of activated carbon. The activated carbon particles have an average grain size of from about 0.4 to about 10 millimeters.
U.S. Pat. No. 4,970,128 by ltoh et al. discloses a supported platinum alloy electrocatalyst for an acid electrolyte fuel cell. The platinum alloy includes platinum, iron and copper. The electrocatalyst has better initial activity and lifetime than conventional platinum or other multi-component alloy electrocatalysts. U.S. Pat. No. 5,489,563 by Brand et al. discloses a platinum/cobalt/chromium catalytic alloy which is precipitated onto a carbon support from nitrate salts.
U.S. Pat. No. 4,970,189 by Tachibana discloses a porous, metal-containing carbon material which includes fine particles of a metal having an average particle size of 1 .mu.m or less dispersed in a carbonaceous body. The method includes mixing a metal oxide with an organic, carbonizing and converting the oxide to metal particles. The catalyst includes from about 5 to 50 weight percent metal.
U.S. Pat. No. 5,068,161 by Keck et al. discloses an electrocatalytic material suitable for use in phosphoric acid fuel cells. The material includes an alloy of platinum with another element such as titanium, chromium, manganese, iron, cobalt, nickel, copper, gallium, zirconium or hafnium. The platinum alloy loading is 20 to 60 weight percent and the electrochemical area of the alloy is greater than about 35 m.sup.2 /g.
U.S. Pat. No. 5,120,699 by Weiss et al. discloses a catalyst containing from 0.01 to 5 weight percent platinum on a graphite support. The graphite support has a particle size distribution of from about 1 to 600 .mu.m. The catalyst material has good longevity when used for hydrogenation reactions.
U.S. Pat. No. 5,453,169 by Callstrom et al. discloses an electrocatalytic material including glassy carbon which contains graphite crystals having a size of from about 1 to 20 nanometers.
U.S. Pat. No. 5,501,915 by Hards et al. discloses a porous electrode suitable for use in a solid polymer fuel cell which includes highly dispersed precious metal catalyst on particulate carbon which is impregnated with a proton conducting polymer.
The foregoing methods generally result in poor control over the composition and microstructure of the electrocatalytic materials, as well as the dispersibility and surface area of the metal on the carbon surface. Further, alloy compositions such as platinum/ruthenium used for oxygen reduction in a fuel cell are not made in a reproducible fashion. The inability to control the fundamental powder characteristics is a major shortcoming for the future development of the electrocatalyst materials.
In addition to electrocatalyst applications metal-carbon composite powders are also useful for electrically and thermally conductive traces in microelectronic applications. Such traces are typically formed using a thick-film paste. The resulting traces have good flexibility when fired at low temperatures and are useful for many applications, including touch screens and similar devices.
It would be advantageous to provide a flexible production method capable of producing metal-carbon composite powders which would enable control over the powder characteristics as well as the versatility to accommodate metal-carbon compositions which are either difficult or impossible to produce using existing production methods. It would be advantageous to provide control over the particle size, particle size distribution, weight loading of the metal and carbon, surface area of the powder, pore structure of the powder and compositional uniformity. It would be particularly advantageous if such metal-carbon composite powders could be produced in large quantities on a substantially continuous basis.